U.S. patent number 4,564,935 [Application Number 06/569,644] was granted by the patent office on 1986-01-14 for tropospheric scatter communication system having angle diversity.
This patent grant is currently assigned to The United States of America as represented by the Secretary of the Air. Invention is credited to Philip D. Kaplan.
United States Patent |
4,564,935 |
Kaplan |
January 14, 1986 |
Tropospheric scatter communication system having angle
diversity
Abstract
A tropospheric scatter communication system provides diverse
angle transmission paths which may be utilized alone or in
conjunction with frequency and/or other known diversity system
arrangements for improved performance. The diverse angle
transmission paths are routed through sum and difference
"monopulse" beams displaced in azimuth.
Inventors: |
Kaplan; Philip D. (Nashua,
NH) |
Assignee: |
The United States of America as
represented by the Secretary of the Air (Washington,
DC)
|
Family
ID: |
24276264 |
Appl.
No.: |
06/569,644 |
Filed: |
January 10, 1984 |
Current U.S.
Class: |
370/339; 342/373;
342/381; 455/276.1 |
Current CPC
Class: |
H04B
7/22 (20130101); H04B 7/10 (20130101) |
Current International
Class: |
H04B
7/02 (20060101); H04B 7/10 (20060101); H04B
7/22 (20060101); H04B 007/08 () |
Field of
Search: |
;343/373,380,381,382,383,384,853 ;455/137,273,276,277,278 ;370/38
;375/100 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Orsino, Jr.; Joseph A.
Attorney, Agent or Firm: Singer; Donald J. Donahue; Richard
J.
Government Interests
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or
for the Government for governmental purposes without the payment of
any royalty thereon.
Claims
What is claimed is:
1. A tropospheric scatter communication system having diverse angle
and diverse frequency signal transmission modes of operation
comprising:
a dual feed antenna system having first and second feed elements
whose phase centers are displaced in the azimuthal plane;
a first and a second preselector unit;
a first duplexer for coupling signals of frequency f.sub.1 from a
first transmitter to said first feed element and for coupling
signals received by said first feed element to said first
preselector unit;
a second duplexer for coupling signals of frequency f.sub.2 from a
second transmitter to said second feed element and for coupling
signals received by said second feed element to said second
preselector unit;
said first and said second preselector units each separating
signals of frequencies f.sub.3 and f.sub.4 received thereby;
first and second hybrid units each having first and second inputs,
a signal sum output and a signal difference output;
means for coupling signals of frequency f.sub.3 from said first
preselector unit to the first input of said first hybrid unit;
means for coupling signals of frequency f.sub.4 from said second
preselector unit to the first input of said second hybrid unit;
means for coupling signals of frequency f.sub.4 from said first
preselector unit to the second input of said second hybrid
unit;
means for coupling signals of frequency f.sub.3 from said second
preselector unit to the second input of said first hybrid unit;
first and second receiver means having their inputs coupled to the
signal sum and signal difference outputs respectively of said first
hybrid unit;
third and fourth receiver means having their inputs coupled to the
signal sum and signal difference outputs respectively of said
second hybrid unit; and
a signal combiner having its inputs coupled to the outputs of said
first, second, third and fourth receiver means and providing a
single output signal related to the signal of maximum amplitude
applied to its inputs.
2. A tropospheric scatter communication system having diverse angle
and diverse frequency signal paths comprising:
an antenna system having first and second feed elements whose phase
centers are displaced in the azimuthal plane;
a first and a second preselector unit coupled to said first and
second feed elements respectively;
said first and said second preselector units each separating
signals of frequencies f.sub.3 and f.sub.4 received thereby;
first and second hybrid units each having first and second inputs,
a signal sum output and a signal difference output;
means for coupling signals of frequency f.sub.3 from said first
preselector unit to the first input of said first hybrid unit;
means for coupling signals of frequency f.sub.4 from said second
preselector unit to the first input of said second hybrid unit;
means for coupling signals of frequency f.sub.4 from said first
preselector unit to the second input of said second hybrid
unit;
means for coupling signals of frequency f.sub.3 from said second
preselector unit to the second input of said first hybrid unit;
first and second receiver means having their inputs coupled to the
signal sum and signal difference outputs respectively of said first
hybrid unit;
third and fourth receiver means having their inputs coupled to the
signal sum and signal difference outputs respectively of said
second hybrid unit; and
a signal combiner having its inputs coupled to the outputs of said
first, second, third and fourth receiver means and providing a
single output signal related to the signal of maximum amplitude
applied to its inputs.
3. A tropospheric scatter communication system having diverse angle
and diverse frequency signal paths comprising:
an antenna system having first and second feed elements whose phase
centers are displaced in the azimuthal plane;
first and second preselector means coupled to said first and second
feed elements respectively;
said first and said second preselector means each separating
signals of frequencies f.sub.3 and f.sub.4 received thereby;
first means coupled to said first and said second preselector means
for providing sum and difference signals of signals of frequency
f.sub.3 ;
second means coupled to said first and said second preselector
means for providing sum and difference signals of signals of
frequency f.sub.4 ;
and combiner means coupled to said first means and said second
means for selecting the signal of greatest amplitude of said sum
and difference signals.
4. A tropospheric scatter communication system having diverse angle
signal transmission paths comprising:
a reflector type antenna having a pair of feed elements of
substantially identical geometry and signal response
characteristics and whose phase centers are displaced in the
azimuthal plane;
a hybrid unit having a pair of inputs coupled to respective ones of
said pair of feed elements, a signal sum output and a signal
difference output;
and means coupled to the signal sum output and the signal
difference output of said hybrid unit for selecting the signal of
greatest amplitude at said signal sum output and said signal
difference output.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a tropospheric scatter
communication system, and more particularly, to a tropospheric
scatter communication system having an angle diversity response
capability.
Tropospheric communication has emerged from its uncertain
beginnings in the early nineteen fifties to become a robust
communication system that fills a gap between line-of-sight
microwave links and long range HF or LF links. Tropospheric forward
scatter occurs in the region between the stratosphere and the
earth's surface in the presence of "blobs" of atmosphere having
refractive index variations. Such variations are the result of
differences in temperature, pressure and gaseous constituents, the
main variable being water vapor. When irradiated by microwave or
UHF signals the blobs re-radiate the signals in all directions,
some of which scatter in the forward direction to produce
electromagnetic fields at the receiving location.
The collection and analysis of empirical data from experimental and
operational tropospheric scatter sites characterize its statistical
performance in terms of short term and long term amplitude-time
distributions. Short term distributions, measured over intervals of
tens of seconds, describe a Rayleigh distribution, from which
hourly median values are obtained. Long term distributions
represent the variation of the hourly median values over longer
periods of time, a month a season or year, and vary considerably
with the season of the year and with geographical location.
Methods of coping with short term (Rayleigh) fading have been
devised through the use of "diversity" transmission paths; paths
that are independent and therefore afford greater reliability than
a single transmitter/receiver at each end. Effectively proven
methods employ space diversity, (two or more antennas spaced
approximately 200 wavelengths apart), frequency diversity, (two or
more carriers separated in the MHz), polarization diversity, and
time diversity (repetition of the same information when slow data
rates are involved).
The use of any one method may be utilized independently in dual
diversity, or may be combined judiciously in quadruple diversity.
In many operational systems, space and frequency diversity are
combined to provide reliable communication links. However, since
two widely separated antennas are required for space diversity,
this necessitates a large communications site. Furthermore,
substantial costs are involved in the construction of the remote
second antenna, its mounting base, and the transmission of signals
thereto.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
tropospheric scatter communication system of improved performance
and reduced cost.
It is another object of the present invention to provide a
tropospheric scatter communication system having angle
diversity.
It is a further object of the present invention to provide a
tropospheric scatter communication system having both diverse angle
and diverse frequency signal paths.
The angle diversity system disclosed herein provides an independent
(dual diversity) transmission path having desirable cost saving
features and which when combined with frequency diversity (quad
diversity) or additionally with polarization or time diversity
(eight fold diversity) provides increasingly higher system
reliability.
In the angle diversity mode described herein, alternate angle
transmission paths are routed through sum and difference
"monopulse" beams formed by a dual-feed antenna whose feed elements
are displaced in azimuth. The received sum and difference signals
are compared at the receiver site and the signal having the
greatest amplitude is selected for further processing and use.
BRIEF DESCRIPTION OF THE DRAWINGS
The foregoing and other objects of the invention will be more
clearly understood from the following description and accompanying
drawings, in which:
FIG. 1 is a functional block diagram representation of the present
invention;
FIGS. 2A and 2B are illustrations of the vertical and azimuthal
antenna pattern linking relationships respectively of a pair of
communication sites utilizing the present invention; and
FIG. 3 is a graph depicting the transmitter and receiver azimuthal
plane antenna gain patterns of a single site.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 is a functional block diagram of the quadruple
(angle/frequency) diversity implementation of the present
invention. A dual feed antenna 2 having feed elements 4 and 6 with
displaced phase centers in the azimuthal plane, simultaneously
serves a pair of transmitters, denoted transmitter A and
transmitter B, and four monopulse receivers 6A-6D. Transmitters A
and B and receivers 6A-6D are isolated through duplexers 8 and 10.
Feed elements 4 and 6 are connected to duplexers 8 and 10 via feed
lines 12 and 14 respectively. Transmitters A and B operate at
"diverse" frequencies f.sub.1 and f.sub.2 and their output signals
are applied via their respective output leads 16 and 18 to
duplexers 8 and 10 respectively and thence to their respective feed
elements 4 and 6. Thus one half of the available aperture of
antenna 2 in the azimuthal plane transmits at frequency f.sub.1 and
the other half transmits at frequency f.sub.2.
On receive, signals at two frequencies, f.sub.3 and f.sub.4, arrive
at both of the antenna feed elements 4 and 6, pass through
duplexers 8 and 10 and preselectors 12 and 14, to form separated
signals of frequencies f.sub.3 and f.sub.4 at the outputs of each
of the preselectors 12 and 14. These signals are then combined in
sum and difference hybrid circuits 20 and 22 and yield the
quadruple diversity channels .SIGMA.(f.sub.3), .DELTA.(f.sub.3),
.SIGMA.(f.sub.4), .DELTA.(f.sub.4) for application to the four
receivers 6A-6D. The four channel receiver outputs are then
"combined" in a post detection combiner unit 24 using conventional
circuitry and methods.
The preferred combining process entails individual amplitude
detectors associated with each of the quadruple diversity channels.
The channel exhibiting maximum amplitude at any given time is
selected for further information processing, while the other three
channels go unused. This preferred post detection combining process
offers greater reliability than predetection combiners, with nearly
identical signal to noise ratios.
FIGS. 2A and 2B, illustrate the transmitter and receiver antenna
pattern relationships in the vertical and azimuthal planes
respectively. In the vertical plane the scattering volume 30 is
seen at the intersection of the transmitter and receiver beams. The
relationships between the sum (.SIGMA.) and difference (.DELTA.)
beam patterns in the azimuthal plane are shown in FIG. 2B. For the
.SIGMA. pattern the scattering volume resides directly above the
great circle path joining the transmitter and receiver. For the
.DELTA. pattern, the scattering volume resides on either side of
the .SIGMA. scattering volume.
FIG. 3 illustrates the azimuthal plane antenna gain patterns at a
single communications site. It will be noted that the .DELTA.
Receive antenna pattern is symmetrically disposed on either side of
the .SIGMA. Receive pattern. Further, it is included within the
angle occupied by the Transmit pattern and that each has
approximately one half the power of the .SIGMA. Receiver
pattern.
The utilization of angle diversity as an alternate transmission
path through the generation of "monopulse" antenna beams provides a
performance advantage over space diversity, as seen in the
following example:
DEW Line tropospheric scatter sites utilize pairs of 60 foot
reflector dishes separated by approximately 250 feet to achieve
space diversity in combination with frequency diversity. By design,
the signals that arrive at the antennas are uncorrelated and
accordingly the transmitter gain, receiving aperture, and beamwidth
(nominally 1.5.degree.) are governed by the individual antenna size
(normally 60 feet).
As an alternative, the angle diversity implementation described
herein would mount two 60 foot dishes side-by-side. In such
proximity, the signals at the two feeds are essentially correlated.
Thus the receiving aperture is essentially doubled; the .SIGMA.
azimuthal beam approaches 0.75.degree., and the beams approach
1.5.degree.. The emitted radiation power, however, would be akin to
the space diversity case, as the two half apertures are emitting
separate frequencies. Signal to noise improvement may therefore
approach 3 dB.
Furthermore, single installation of the massive antenna mounts
would appear to be less costly and the construction more
conveniently maintained than the alternative. In particular, as the
tropo sites serving the DEW Line operate as relay links, the case
for single antenna installation is doubly advantageous.
Although the invention has been described with reference to the
preferred embodiment thereof, it will be understood to those
skilled in the art that the invention is capable of a variety of
alternative embodiments within the spirit and scope of the appended
claims. If, for example, it is desired to provide a system having
only angle diversity (dual diversity), the system would utilize
only one of the pair of feed elements 4 or 6 for transmitting the
single frequency. This is the case since the use of both of the
feed elements during transmission would produce a narrow
transmitter beam, incapable of linking with the .DELTA. Receive
beams at the receiver site. For signal reception however, the
single frequency signal from each feed horn would be combined as
before to produce the sum and difference signals thereof.
* * * * *